1. Field of the Invention
The present invention relates to an apparatus for laminating multilayered printed circuit boards and more particularly, to the use of a high pressure sealing ring into which a heated gas may be introduced for laminating multilayered printed circuit boards.
2. Prior Art
Multilayered printed circuit boards comprise a number of circuit layers separated by insulation layers which are laminated together to form a solid board. Each circuit layer consists of an insulation layer on which copper circuit traces appear on one or both sides thereof. The insulation layer is, typically, a thin sheet of fiberglass or film dielectric and such insulating sheets are placed between the circuit layers.
In the past, such multilayered printed circuit boards have been laminated together by being placed between a pair of steel plates within a hydraulic press. The press is heated to a predetermined temperature and a pressure is applied for a predetermined time. The multilayered assembly may be placed within an air tight, sealed chamber which is connected to a vacuum pump whereby air bubbles and gases produced during the heating of the material may be removed.
This prior art method has become unsatisfactory for at least two reasons. In the past, multilayered printed circuit boards have either been rigid or flexible. In a rigid circuit board, each circuit layer consists of an insulation layer made from a rigid material. In a flexible circuit board, each circuit layer consists of an insulation layer made from a flexible sheet of material. However, modern multilayered printed circuit boards have both rigid and flexible portions. The lamination of such boards presents a number of difficult technical problems relating to thermal expansion and the even application of pressure during the lamination process. The portions of the multilayered circuit board which are rigid have coefficients of thermal expansion that are different than the portions which are flexible. Furthermore, the portions which are flexible are typically much thinner than the rigid portions so that elaborate inserts must be developed so that the press applies a uniform pressure to the entire circuit board during the lamination process.
Another problem relates to the spacing between the copper circuit traces on the individual circuit layers. Modernly, the circuit traces have become finer and more closely spaced and it is necessary that the insulation material flow uniformly into the spaces between all circuit traces during the lamination process. In this manner, the insulating layer functions not only to insulate the circuit traces on one layer of a multilayered board from the facing traces on an adjacent layer, but also serves to insulate the individual traces on each multilayered board from each other. The steel plates of conventional hydraulic presses, being rigid, will not conform to irregularities in the multilayers and are simply not capable of applying a uniform pressure which will cause the insulation material to flow uniformly into all of the spaces between the circuitry. Therefore, air becomes trapped within the layers. This air creates spaces between circuit tracers which are not filled with insulation material, resulting in possible shorts and rendering the multilayered printed circuit board subject to possible failure during operation.
One plausible solution to the aforementioned problems of the prior art has been provided by a invention of the applicant of the present application in an application entitled METHOD AND APPARATUS FOR LAMINATING MULTILAYERED PRINTED CIRCUIT BOARDS HAVING BOTH RIGID AND FLEXIBLE PORTIONS filed on Mar. 9, 1984 under Ser. No. 587,928. This prior patent application of the present applicant is incorporated herein by reference. In that prior application a pressure vessel is disclosed for laminating the multiple layers of a multilayered printed circuit board by employing heating and cooling elements within the vessel. The multilayered circuits are placed on a steel plate which rests on a bleeder plate which, in turn, rests on a vacuum plate. The circuit layers are covered with another steel plate, a sheet of silicone material, a breather blanket and a plastic vacuum bag which is hermetically sealed to the plate using a silicone sealing material. The vacuum line is connected from the vacuum plate out of the vessel to a vacuum pump. The printed circuit boards and the enclosure are placed in the pressure vessel and the vessel is sealed. An inert gas, such as nitrogen or carbon dioxide is introduced into the pressure vessel to achieve the desired pressure. Then the vessel is heated to a suitable lamination temperature. While the materials are being heated, any gases given off are removed by the vacuum pump. After outgassing is completed and the temperature in the pressure vessel is raised to the required level, the induced high pressure is maintained for a predetermined period of time after which the heating element is deactivated and the cooling is activated to cool down the multilayered circuit board while the pressure is maintained. The pressure is then released and the gas expelled from the pressure vessel which may then be opened to remove the parts.
Although the aforementioned prior patent application of the present applicant discloses a novel and substantially advantageous advance over the prior art for laminating the various layers of a multilayered printed circuit board, it has been found that in typical applications, up to 5,000 gallons of inert gas such as carbon dioxide is necessary to accomplish the lamination process. This volume of gas is itself a disadvantageous expense. Furthermore, the requirement for filling a pressure vessel and then evacuating a pressure vessel with 5,000 gallons of carbon dioxide or other inert gas is a time consuming process and therefore an additional costly feature of the prior invention. Finally, the process of heating up to 5,000 gallons of an inert gas to the requisite temperature for the lamination process can be quite costly from the standpoint of energy utilization and therefore also adds significantly to the overall expense of utilizing the applicants' previously disclosed process and apparatus.
As noted in the prior aforementioned application of the present applicant, it is known in the prior art to use air pressure to laminate two parts together. The two parts to be laminated together are typically placed in a vacuum bag within a vessel into which air pressure is introduced. The air within the vessel is heated to a predetermined temperature and pressure is applied for a predetermined time. However, while lamination of two parts using air pressure has been known, it has never been thought possible heretofore that such a technique could be used to perform the intricate lamination required when laminating a large number of complex layers of a multilayered printed circuit board.